Add refyner plugin (second-brain research + capture)

[dragos-rebegea]

Summary

• Vendors the refyner Claude Code plugin at repo root (./refyner), matching the existing local-plugin convention (e.g. connect-apps).
• Bundles the Refyner MCP server (OAuth, https://refyner.me/api/mcp) and the second-brain skill, which grounds research in the user's saved Refyner knowledge and supports propose→confirm capture.
• Adds a refyner entry to .claude-plugin/marketplace.json under the integrations category.
• Adds a one-line entry to the README's Integrations section.

About Refyner

Refyner (https://refyner.me) is a personal "second brain" app. This plugin lets Claude Code research grounded in what a user has already saved to Refyner, and propose new captures back into it (with explicit user confirmation before anything is written). Requires a free Refyner account; the bundled MCP server authenticates via OAuth.

Source of truth for the plugin: https://github.com/buidly/refyner-claude

Verified

• .claude-plugin/marketplace.json parses as valid JSON (python3 -m json.tool).
• Plugin directory structure matches the existing local-plugin convention.

Not verified (maintainer to confirm)

• End-to-end install + MCP OAuth connection in a live Claude Code session.